Materials Performance

NOV 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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45 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 Nuclear power plants in the United States account for ~20% of this country 's electric generating capacity. 3 As most of these plants individually contribute a large volume of electricity to the grid and main- tain high-capacity factors, they are consid- ered the first facilities dispatched to supply the grid to cover known, high-reliability demand. 3 Figure 1 shows the present distri- bution of nuclear power production facili- ties supporting public demand. It should be noted that nuclear power plants built to this capacity and reliability require significant levels of capital invest- ment (>$8 to 10 billion, U.S.). 4 This is greater than first costs for other generating sources of similar power output, but results in much higher reliability and survivability in the face of environmental events. Such design and performance factors are part of plant operating licenses, issued by the Nuclear Regulatory Commission (NRC) intended to support safe electricity production for a fixed life. This investment and impact war- rant a comprehensive approach to asset in- tegrity management to ensure that safety, reliability, and cost performance are judi- ciously defended from corrosion threats. If these measures are effective and economic, operators may legally extend the permitted life of a plant from 40 years to 60 or even 80. 5 Experimental Procedures Nuclear power plants abide by a strong industry culture for developing, implement- ing, managing, and evaluating programs to proactively meet operating commitments by sustaining corrosion performance. Pro- grams satisfy expectations for regulatory compliance, reliability, and economic suc- cess through: • Risk-based inspection (RBI) of piping, pressure vessels, tanks, and other structures • Remaining life assessments to dem- onstrate operational viability • Corrosion control measures for inter- nal and external effects • Str uctural h ealth monitorin g for long-term trending • Mitigating failure consequences for low-margin components • Long-term asset integrity manage- ment investments Plants manage f low-accelerated, raw water, and soil-induced corrosion effects using RBI programs—a combination of probability and consequence guides where and when to inspect. However, it is the con- sequence of the failed component, either by leakage, structural failure, noncompliance, or isolation to complete repairs, that is the governing factor in most programs. If this consequence is intolerable (e.g., contami- nating groundwater), the facility is likely to take proactive action at a much lower prob- ability of occurrence. Operators use predic- tive models weighing inputs for corrosion- driving variables and correct them with empirical data from prior inspections, repairs, or leaks to achieve high-value results. These methodologies and tools are often developed via collaborative initiatives through utility participation and invest- ment into the Electric Power Research Insti- tute (EPRI). An example of this process is the indus- try's flow accelerated corrosion (FAC) pro- grams to address erosion and corrosion- induced failures identified under Generic Letter 89-08 (GL 89-08). 6 FAC is specifically ascribed to single and multiphase flow dur- ing normal, start-up, and upset conditions, but generally isolated to steam, condensate, and feedwater piping systems, including vents and drains. The process significance and risk for hazardous potential energy make FAC a significant factor in a site's cor- rosion control programs, as evident by tragic events at Surry and Mihama Power Stations in 1986 and 2006, respectively. 7-8 Figure 2 demonstrates how FAC's distur- bance of surface oxide films altered surface geometry/f low effects and led to acceler- ated failure. Continued monitoring offers stations advanced warning prior to failures, especially where similar conditions and processes can result in comparable wear rates, even at different power plants. This enables the industry to develop and lever- age extensive, empirically derived data- bases and models to manage this mecha- n i sm p r o a c t iv e ly a n d a d d re s s s a f e ty concerns targeted by GL 89-08. Predictive tools applied to long-lived nuclear power plants enable investments made for inspections, corrosion control, structures monitoring, consequence miti- gation , and replacement/mitigation to receive clear economic and risk assessment FIGURE 2 Flow-induced corrosion mechanism at orifice plate. 9

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